Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor 13 compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
Gas turbines, and particularly aircraft gas turbines, are relatively high capital cost assets having relatively high value to the customer. Consequently, it is desirable to maintain such gas turbines in situ, such as in a power plant or on the wing of an aircraft, without having to disassemble major assemblies of the gas turbine engine. Such maintenance activities may include removal of a damaged area of a component such as compressor and stator blades of the compressors 13, 14. One method for removing damaged areas while the engine is installed is to mount a laser ablation tool to a flexible or rigid shaft which is inserted into an engine or through a suitable inspection port. The laser ablation tool is then used to ablate a surface of the component to be repaired to remove an area of the component. Such a process is known within the art as “bore blending”.
Laser ablation comprises directing a beam of high intensity coherent electromagnetic radiation at a target such as a metal, such that the metal undergoes a phase change from solid directly to gas without undergoing an intermediate liquid phase (i.e. ablation). One method is to use a pulsed laser. US patent US 20120121382 describes one known method of carrying out pulsed laser ablation bore blending.
Unfortunately use of scopes has limitations in terms of their action. It will be understood that steering the distal end to which the laser ablation tool is mounted requires manipulation of the shaft. It is difficult to steer a shaft with an overall length of action which is greater than approximately five meters.
In particular, there are a limited number of access points into an engine and in such circumstances it is quite common to require an abrupt change in direction of the scope in order to gain access to the desired observation point, which may create severe difficulties with regard to jamming of the tool. One component which has been found to be particularly difficult to access using conventional arrangements is the 6th stage of the high pressure compressor 14 (HP6). This is particularly problematic, as the HP6 compressor has been found to be subject to damage in use at a relatively high rate.
Due to the difficulty of access for bore blending, the component may not be accessible at a preferred angle for conducting laser ablation. For example, access may only be available from a face of a compressor blade or stator (i.e. a surface extending between the leading and trailing edges of the blade or stator), rather than from other angles as might be desired. Furthermore, it is necessary to prevent pieces from the damaged component from being detached and falling into the engine, where it might cause damage, known as “domestic object damage” (DOD).
A further problem associated with bore blending using laser ablation is the creation of “secondary damage” due to residual effects such as the formation of a heat affected zone (HAZ) on the component by the laser ablation. A HAZ is an area of a metal which has been subjected to high temperatures below the melting point of the metal, but above a critical temperature which may alter the bulk properties of the metal, such as areas adjacent an ablated area of the component. The metal within a HAZ is weakened relative to the bulk material, such that the area adjacent the removed material may have insufficient strength subsequent to the repair. Furthermore, the high temperatures to which the metal within the HAZ is subjected may cause chemical changes to the metal, such as oxidation, which may be undesirable. Any repair performed by conventional laser ablation techniques may therefore be of reduced quality compared to the original component. Alternatively, the laser ablation may have to be carried out at a relatively low “fluence” i.e. radiative flux of the beam integrated over time, resulting in a relatively long process time for ablating a given volume of material.
The present specification describes a repair method which seeks to overcome some or all of the above problems.